In a large mixing bowl, cream the butter and the sugar until light and fluffy. Blend in the egg yolks.
In a small bowl, whisk together oil, cocoa, and vanilla until smooth. Combine the cocoa mixture with the butter mixture, scraping the sides of the bowl, until completely incorporated.
Add the flour and salt to the wet ingredients, blending with a mixer until combined. Refrigerate cookie dough for 30 minutes to firm up.
Preheat oven to 350°F Scoop out balls of cookie dough with a small ice cream scoop and place 1-inch apart on parchment-lined cookie sheets.
Bake for 12 to 15 minutes, until a toothpick inserted in the cookie comes out clean.
After removing cookies from the oven, use a rounded soup spoon to gently press an indention or puddle in the center of each warm cookie. Let cool completely.
To make the chocolate ganache:.
Slowly melt chopped semisweet chocolate in the microwave. Add cream and whisk vigorously until smooth and thick. Place in a zip-top bag, squeeze out the air, and seal. Cut a small corner of the bottom of the bag.
Pipe the chocolate ganache into the center puddle indentation of each cookie. If you have any leftover chocolate, use it to drizzle across the outer edges of the cookies. Cool completely before serving.